You are running Firefox 60, according to your stats on the forum. This version didn't automatically get the fix for the issue with the add-ons.

You need to download this extension to fix the issue, if you have not done so already.

Thanks for that reply. I was confused by the fact that some extensions installed, but not Bitdefender.

pccobbler said

Before Mozilla's add-ons bug, I could install Bitdefender TrafficLight. Now, however, I receive the error message "Download failed. Please check your connection." Please fix this bug.

Not a bug in Any Firefox version per say.

What happen was an unfortunate situation where an essential certificate used to sign many extensions expired.

https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/

Mozilla did provide updates for the Firefox 66.0 Release and the 60.0 ESR.

So 66.0.4/66.0.5 and 60.6.2esr/60.6.3esr has had certificates fixes.

Your Linux distro has likely had Firefox package updates for these versions for a while now as 66.0.5 and 60.6.3esr has been available since May 8th.

Wesley Branton said

You are running Firefox 60, according to your stats on the forum. This version didn't automatically get the fix for the issue with the add-ons. You need to ... to fix the issue, if you have not done so already.

Firefox versions since the 16.0.2 Release has not shown the minor version in UserAgent.

Many LTS Linux distros tends to provided Firefox versions on ESR channel which is currently at 60.6.3esr. The UA for 60.6.3esr shows as if you have 60.0. The OP is likely perhaps using 60.6.1esr or older and not updated to 60.6.3esr or even 60.6.2esr to be getting this error.

Actually if the user has changed the fingerprint preference from default one of the things it does is change the UA to appear as current ESR, in this case Firefox 60.0 ESR